1. Field of the Invention
The present invention relates to the field of semiconductor processing and, more particularly, to run-to-run control in semiconductor processing.
2. Description of the Related Art
Today there are many new and innovative manufacturing processes. In semiconductor manufacturing processes, innovative processes are hindered due to a large number of important steps in producing a semiconductor device. Each step in these processes is generally important to the final product. Moreover, the various processes, from the initial growth of the semiconductor material, the slicing of the semiconductor crystal into individual wafers, the fabrication stages (etching, doping, ion implanting, or the like), to the packaging and final testing of the completed device, are so different from one another and specialized that the processes may be performed in different manufacturing locations that contain different control schemes.
Some of the important processes include chemical-mechanical-planarization (CMP), lithography, including overlay and critical dimension (CD) and plasma etching. Overlay and critical dimension are two of several important steps in the lithography stage of semiconductor manufacturing. Overlay control involves measuring the misalignment between two successive patterned layers on the surface of a semiconductor device. Measurements needed for overlay control, such as the x-offset, y-offset, magnification, skew and rotation can be determined initially from a calibration run. However, measurement of the errors in these variables are not measured until after the development of a resist. The resist development is a separate process that takes place long after the lithographic exposure. Overlay control is not the only process for which measurements important to the process are completed well after the process run. Furthermore, other processes outside the semiconductor processing industry also require measurements after a completion of the run of a process.
Due to the separation in the measurements of vital and relevant product characteristics versus the actual process that affects the measurements, measurements are often taken at a different time from when the materials are processed. Thus, the measurements are asynchronous from the process. As a result, there is an accumulation of varying numbers of measurements, on a plurality of different products, that are out of sequence from the order in which the products were processed. Furthermore, some measurements in the sequence are not available at the time a process tool requires a new process setting for a new lot because an operator measured material out of order. In other circumstances, an operator may not have completed a full group of lots queued for measurement at the time a request for a new process setting occurs.
Prior art approaches to the dilemma of later acquired measurements include ignoring missing measurements and imposing operating restrictions on processing such as delaying further processing until measurements are available. Neither approach is satisfactory. Ignoring missing measurements degrades control performance by making what may be vital information unavailable to a controller. Delaying processing reduces manufacturing throughput due to the idle time of waiting for measurements.
What is needed is a more efficient and accurate manufacturing system and method that can account for later acquired measurements for process settings.
In accordance with the present invention, a run-to-run (R2R) control system provides efficient and accurate process controls that apply a same process for a first run of the control system and one or more subsequent runs. More particularly, the method for controlling a manufacturing process includes: processing materials using a process input and producing a process output; storing the process input in a database, the storing including using a timestamp; storing at least one measurement of the process Output in the database aligned with each process input using the timestamp; iterating over the data from the database to estimate a process state; and, if one or more of the measurements is missing from the database, predicting the missing measurements for the database based on a model, and determining an error for calculating a next process input, the error based on the data in the database.
According to one embodiment, the method includes one or more modules coupled to the database, the one or more modules including at least a sorting module configured to sort measurements received asynchronously from the process according to the timestamp, the sorted measurements including later arriving measurements to allow the next process input to be based at least in part on an error calculated using later available measurements.
Another embodiment is directed to a control system, including: at least a controller for providing a process input; a process tool coupled to the controller to receive the process input and provide a process output; a database coupled to receive the process input and a measure of the process output, the database aligning each process input and each process output according to a timestamp process; a model coupled to receive a database output and provide a model output; and a state estimator coupled to receive the database output and the model output, the state estimator coupled to the controller to provide estimated process inputs based on estimated measurements of the process output corresponding with the aligned process input and process output.
According to another embodiment, the database output includes measurements performed asynchronously from the process, and missing measurements are inserted in an order based on the timestamp process. Further, the database stores the measure of the process output and the estimated measurements of the process output according to a receding horizon filter and the timestamp.